Ultra high frequency triode converter



E. W. HEROLD ET AL ULTRA-HIGH FREQUENCY TRfODE CONVERTER Feb. 3, 1948.

Filed Jan. 26, 1944 um um v .5950 VHN E m m. N W0 gun 0 Dr RH. T 1 mm WW Y B ISAFDO EH Patented Feb. 3, 1948 ULTRA HIGH FREQUENCY TRIODE CONVERTER Edward W. Herold, Kingston, and Dwight 0. North, Cranbury, N. J assignors to Radio Corporation of America, a corporation of Delaware Application January 26, 1944, Serial No. 519,730

Claims.

The present invention relates in general to ultra-high frequency converter systems, and more particularly to such systems utilizing a triode tube of special construction whereby more efiicient frequency conversion and improved signalto-noise ratio are obtained in the reception of signals by the superheterodyne method.

The problem of frequency conversion in' a superheterodyne radio receiver has received considerable attention during the last ten years. Until recently. it was believed that tubes for'this purpose might always be rather complicated multi-grid structures. However, the high fluctuation noise inherent in such structures has led to a return to simpler tube types such as the trlode and the pentode in an attempt to obtain better signal-to-noise ratio. This is particularly true at ultra-high frequencies at which the attainment of large transmitter power is diflicult and signals may consequently be weak. It is well known that a triode tube has less fluctuation noise thana pentode of comparable structure but it suffers from feedback effects which are serious even in converter operation.

It is, therefore, one of the main objects of our present invention to provide improved circuits using special triode tubes for converter operation and wherein feedback efl'ects are greatly reduced, particularly at ultra-high frequencies. 7 Another object of the invention is to provide the cathode and plate of the converter tube each with a pair of leads which have connected to them suitable external elements for eliminating detrimental feedback effects.

Another object is to provide a high frequency converter system utilizing a tube having a control grid, a cathode, and an anode provided with two leads, and wherein the output circuit tuned to the intermediate frequency is connected to one of the anode leads, and a path of low impedance for signal frequencies but high impedance for intermediate frequencies is connected between the cathode and the anode through the second anode lead.

A further object is to provide, in the high'frequency converter system mentioned above, a low impedance path constituted by anope'n transmis sion line equivalent to an open quarter-wave section at signal frequencies, one conductor of which is connected to the cathode and the other con ductor of which is connected to the second of the anode leads. V

A still further object is to provide, in the high frequency convertersystem mentioned above, a low impedance path constituted by ,a concentric conductor resonant line equivalent to a quarter'- wave open section at signal frequencies, the inner cylinder of which is insulated throughout its length and connected at its near end to the second of the anode leads and the outer cylinder of which is connected to the cathode.

These and other objects and advantages of the invention will be apparent from the following specification whentaken with the accompanying drawings, in which:

Fig. 1 represents a conventional high frequency triode converter circuit which will serve to explain the theory underlying the present invention;

Fig. 2 discloses the circuit of an ultra-high frequency converter stage in accordance with the invention, and

Figs. 3 and 4 are modified forms of the converter stage shown in Fig. 2.

In the triode used in a converter stage as herein contemplated, the signal is applied to the control grid, and a local oscillator voltage is also applied, either to the cathode through a low impedance circuit, or to the control grid. The intermediate frequency output is taken from the plate. Cir,- cuit analysis-shows that an inductive reactance in the cathode circuit will cause a reduced input impedance, so that it is preferable to introduce the local oscillator voltage across the signal input circuit or else keep the effective cathode impedance capacitive in character. The effect of a cathode impedance on the input circuit comes about through the grid-to-cathode internal admittance. In the same way, an impedance in the plate circuit also affects the input impedance through the grid-to-plate capacitance.

We may schematically represent a conventional triode converter at high frequencies by a circuit similar to that in Fig. 1. The triode V is shown by the dotted line block representing the envelope of the tube,.with internal capacitanoes Cgk, Ctr, and Cgp. The lead inductance's are shown as 'Lg; Lk and Lp. The I.-F. output circuit is connected to the plate. When a signal is applied to the control grid and alocal oscillator is applied to the cathode, as shown, the tube operates as a modulator, and an 1-1. current flows in the plate circuit, giving rise ,to an' L-F. output voltage which may subsequently be utilized in the ma ner well known in superheterodyne receivers. in addition, however, there'is an alternating current flow from cathode to plate of the tube at the signal frequency. It is the latter current which gives rise to a signal frequencyvoltage across Cpk, C o, aand Ln 1 important result of these signal frequency voltages lies in the resulting feedback currents flowing through the input signal circuit. To a first approximation the effect of such feedback is as though the input circuit were modified age existing between the cathode and ground, and.

by a signal frequency voltage betweenthe plate and ground. Neither of these voltages is of any advantage in normal converter operation and our invention is directed toward both,

Our invention is based upon the followingconsiderations. If the internal plate-to-cathode ca-t pacitance, Cpk, were large enough, the signal frequency current flowing into the plate of the tube would be bypassed directly to the cathode and could not give rise tolargesignal frequency voltage drops. However, such a large capacitance would seriously reducev the,- I.-F. output circuit impedance and/or band: widthand would merely mitigate one defect while increasing another. It would also appear that, if. a;small series induotance could be placed in, series-with Cpl; so as to resonate at the signal frequency, the signal fre quency currents could be. bypassed,- in. this case, without detriment to the I. -F.. output. Since ine isan-inter-electrode capacitance,.such series tuning is not possible. Apartial improvement is possible by a. series resonance of the circuit I'rp, Cpo, Lpo. As an aid in achieving this, the-.prie mary. Lpo of the Iz-F; transformer: may be so Wound as to. provide a. series resonance at the signal frequency. This is possible since-the signal frequency is usually well above the, firstnatural frequency of the I.F. inductance,.and aseotionalizedwinding oreven proper tube. or circuit leads may offer the. correct; inductive reactance to cancel the. capacitive: coil; reactance; At more effective design would: separately'series-tune the platev to ground and? the cathode to ground to eliminate the chief feedbacksmentioned-t above; In examining the triode. converte1vinthe" light of-these considerations; it: occurred: to -.us that) an evenmore satisfactory method of: providing" a pathfrom plate tocathode; of low impedanceaat' signal frequency. and. high. impedance at intermediate frequency, would: be thedirect. connecev tion of a separate circuit; or its. equivalent, directly-between. these twopoints; Of course, such a circuit could presumably be built inside the tube, but this would not permit. adjustment. to different frequencies andmight not be commercially useful. For these reasons, wehave made usesof a modification inthe tube structurewliereby additional leads are brought out from the cathode and from the plateasshown-in Fig. 2ffor triode tube-V1; One: pair opleads, according. to our invention; is then-.usedeifor the normal;converter connections, as in Fig. 1', whilethe; other pair is used to provides, low impedance path;- series-tuned' at: signal frequency and" open-circuited atintermediatef frequency, from plate to cathode. In this way, there can be no signal frequency currents or=signal frequency' voltages iii-the first pairiof leads due-'toztheplate .current flow and all feedbackeffects are: eliminated," pro vided only that thetwo pairs: of." leads. can be externally. separated byhighimpedance connec-v tions, Wehave found'itipossibleiatocdo this.

Atultraehigh frequenciesethe effect. of aseriese tuned circuits-is most easily: obtained by trans the elimination: of:

mission line circuits. For example, a. pair of insulated uniformly spaced wires of a length just equal to a quarter of the wavelength, behaves as a series-tuned circuit across either end. We make use of such an arrangement in the ultra-high frequency converter stage shown in Fig, 2. In this figure T1, T2 and Tsare; resonant circuits of the concentric transmission line type each comprising an inner conductor I and an outer conductor 2, V1 is the triode converter tube and V2 is, a local oscillator tube each comprising a cathode; 3, a control grid 4 and a plate or anode 5. The cathode 3 of converter tube V1 is provided with'a pair of leads 6, 1 and the plate 5' of this tube, iszprovidedwith a pair of leads 8, 9. The circuitTi, comprising a pair of conductors l 0 and. H which are connected, respectively, to cathode lead land anode lead 8, is an open quarter-wave parallel line at signal frequency for the ,purpose of providing a low impedance path for currents of this. frequency from the platev 5 to the cathode 3. of tube V1. In operation, circuit T1 is tuned to, the signal frequency and serves as an antenna coupling. transformer from the transmission line TA.tO. the. grid 4 of tubevi. Thecapacitance. C1 and resistance R1 serve. as a grid condenser and leak, respectively, toprovide appropriate biasfor thegrid 4 of tube V1. One. cathodeleadli of tube V1 is connected. to a point on the inner conduc tor l. of the local oscillator tuning lines T2- and servesto. impress the local oscillator voltage on the cathode 30f tube Vi. Circuits- Taand T3 and the tubeVerepresentatypical.local oscillator. It is, of, course,.obvious, that the resonanttransmission. lineTi is-preferably adjustablein electrical length totuneto the-desired incoming signal frequency. Adjustmentof electrical length. may. be efiected by. adjustment. of physical length of the line, by adjustment of a. variable. capacitance across the line,,or by any other means. This circuitmay be enclosed.- in, a shielmor, in some instances. may. be! a. concentric linein place. of the parallel wirefline shown.

Another V.-H.-F. converter. circuit incorporating our. inventionis shown in Fig.3. This differs from theprevious circuitin that a local-oscillator (not, shown) I is looselycoupled.- to the signalv in! put. circuit I T1 through. a. small capacitance 02. In this circuit, the cathode 3 of tube.V1 .through lead 1 is connectedtothe outer, grounded conductor l2, of a concentric, quarter-waveline T5; The inner conductor l3of this'lineis an open circuit. at itsfar end and, isconnected to,- the plate 5 of tube V1 at its near end through lead 8,.asshown. T-wo cathode leads are-not essential in this circuit, although,theyare still desirable toreducethe. lead losses. The circuit: of Fig; 3, does .not completely eliminatefeedback effects .in the cathode lead,,however,, since the currents through the low impedance plate-to-cathode path may set up.asignal frequency, voltage. between cathodeand ground.

. The basic purposebehindour invention is to, provide (a) a low impedance path, at signal frequency, between plate and cathode and. (b); a. second and independent low impedance path-bee tween the cathodeandthe low side or them? putcircuit. In Fig, 3, our first purposeisaccoma plished, but thesecond-is-not necessarilyachievedifg-the cathode lead'inductancezis appreciable; A more satisfactory solution: in: eliminating the; cathode lead feedback is shown in the circuit of. Fig. 4:where the circuit Tscorrespondin'g to:-Ts in Fig; 3 is'composedzof three-concentricrelea merits;v namely; aninner: conductors; antinter mediate element or cylinder I5 and an outer cylinder 16. The cathode 3 of V1 is connected to ground through one lead 6; while the second cath- =ode lead I is connected to the intermediate elelength is adjusted to be equivalent to a quarter- 'wave open section, The intermediate cylinder and the outer cylinder 16 are connected electrically at the far end and are adjusted to be electrically a quarter-wave long. By this arrangement, the plate 5 and cathode 3 are connected through the low impedance of theinner two concentric elements 44 and I5 whereas the two cathode leads 6 and 1 are separated by the high impedance shorted quarter-wave line formed by the outer two cylinders l5 and I 6.

It will be apparent that many modifications of our basic idea can be made by the skilled worker in the high frequency field and such variations will have advantages in particular cases. We have chosen to include herein only those arrangements which are believed to illustrate our invention most simply and directly.

- What we claim is:

1. An ultra-high frequency converter system utilizing a tube having a control grid, a cathode and an anode, and in which two leads are provided each for the cathode and anode, characterized in that between one of the anode leads and one of the cathode leads there is connected a series-tuned, low-impedance path for signal frequencies, and further characterized in that the cathode is eifectively connected to ground through the other cathode lead and the anode is connected to output circuit through the other anode lead.

2. An ultra-high frequency converter system as defined in claim 1 wherein the low impedance path is constituted by a two-conductor resonant line approximately one-quarter of a wavelength long.

3. An ultra-high frequency converter system as defined in claim 1 wherein the low impedance path is constituted by a concentric conductor resonant line approximately one-quarter of a wavelength long, the inner conductor of which is an open circuit at its far end and connected to said anode lead at its near end, the outer conductor being grounded and connected to said cathode lead.

4. An ultra-high frequency converter system utilizing a tube having a control grid, a cathode and an anode, and in which two leads are provided each for the cathode and anode, characterized in that between an anode lead and a cathode lead there is connected a series-tuned, low impedance path for signal frequencies, and between the two cathode leads there is connected 9. high impedance path.

5. An ultra-high frequency converter system utilizing a tube having a control grid, a cathode and an anode, and in which two leads are provided each for the cathode and anode, characterized in that for preventing the existence of a '5 signal frequency voltage between anode and cathode there is connected, between an anode lead and a first cathode lead, a path of low impedance for signal frequencies and high impedance for intermediate frequencies, and, for eliminating the flow of signal frequency currents from said anode through the second cathode lead, there is con- 6 nected betweenthetwoxcathode leads a high impedancepath. .71

6. An ultra-high frequency converter system comprising a tube havingacontrol grid, a cathode and an anode, said cathode and anode being provided each with two leads, a signal frequency input circuit connected to' the control grid, at source of locally-generated oscillations connected to one or the cathode leads, an output circuit tuned to the intermediate frequency connected to one of the anode leads, and a series-tuned, low impedance path for signal-frequencies connected between the 7 remaining two anode and cathode leads. l

'7. Anultra-high frequency converter system as defined in claim 6 wherein the low impedance path is constituted by a two-conductor resonant line approximately one-quarter of a wavelength long. l i

8. An ultra-high frequency converter system comprisinga tube having a control grid, a cathode andan'anode, said cathode and anode being provided each with two leads, a signal frequency input circuit connectedbetween the control grid and one of the cathode leads, a source of locallygenerated'oscillations coupledto the control grid, an output circuit tuned to-the intermediate frequency connected to one of the anode leads, and a series-tuned, low impedance path for signal frequencies connected between the remaining anode lead and the other cathode lead.

9. An ultra-high frequency converter system as defined in claim 8 wherein the low impedance path is constituted by a concentric conductor resonant line one-quarter of a wavelength long, the inner conductor of which is an open circuit at it far end and connected to said anode lead at its near end, the outer conductor being con nected at its near end to said other cathode lead. 10. An ultra high frequency converter system comprising a tube having a control grid, 9. cathode and an anode, said cathode and anode being provided each with two leads, a signal frequency input circuit connected between the control grid and the first cathode lead, a source of locallygenerated oscillations coupled to the control grid,

an output circuit connected to the anode through one of its leads, and means for minimizing the flow of signal frequency currents from said anode through any portion of the input circuit and its connecting leads, said means including a transmission line consisting of three concentric cylinders, the outer of which is connected to the first cathode lead, the intermediate cylinder being connected to the second cathode lead, the inner cylinder being equivalent to a quarterwave open section, insulated throughout its length and connected to the anode through the second anode lead, and the intermediate and outer cylinders being connected electrically at the far end and being equivalent to a quarterwave shorted section.

11. A high frequency converter system comprising a tube having a control grid, a cathode, and an anode, said anode being provided with two leads, a signal frequency input circuit connected between the control grid and the cathode, a source of locally-generated oscillations connected to an electrode of said input circuit, an output circuit tuned to the intermediate fre quency connected to one of the anode leads, and a path of low impedance for signal frequencies but high impedance for intermediate frequencies connected between the cathode and the'anodc through the second anode lead.

'1 :12. A hi h frequency-converter system lined in claim 11 wherein the low impedancepath is: constituted. by emotion-transmission line equiv alent: to. an open, quarter-wave section. at: Signal frequencies. one conductor or winch svco n edto the cathode, the other conductor being connected to the second of the-anode leads. I 13.. Afhigh frequency converter fiystelnas definedin claim 11, wherein the; low impedance path islconstituted by 3,1.(10110811121'1CL'0QX1Q11CH21"resonant line quivalent to a quartcrrwave open section at. --signal frequencies, the;- innercylinder of which-is irisulatedthrouehoutsits length and con-- nected at its near end to the second: 01% the an leadsiqthe outer; cylinder being; connected; to the cathode. i

144! A highfrequency superheterodyne convertex circuit utilizing a. tube, havin a, c thod electrode, control grid electrode, anode electrode, and at least: one lead-to each Ofythfise. electrode wherein the cathod lead is onnectedto ground a; s nal input circuit is connected between th control rid. and round, an.- interme iate reque cy circuit: is con ected: be we n the. anode and round. and wh re alow m e a c pa for g a equencies: is; p o d d between anode and ground by: the use of. an intermediate. frequency circuit which has high impedance at ine temnediate frequency but which is; in series resohence with the anodeelectrode lead at the signal frequency; q

15. A high frequency superheterodyne, coner-tor circuit utilizin a t e av nzc.--a cathode and-an associated lead thereto,-a control-e199:- trode and a lead thereto, andan anode with; lead thereto, wherein a signal input circuit-is connected between control grid and ground-an intermediate frequency circuit is connected be tween the anode lead and ground, and wherein a low impedance path at signal frequency is pro, videdbetween anode and-ground and a second low impedance path at signal frequency is provided between cathode and ground, saidlow-impedance paths being obtained by a series reso nance at signal frequency of the intermediate frequency circuit and the anode leadandby a. circuit connected between the cathode leadand ground so as to provide a series resonant path at signal frequency between cathode and ground.

EDWARD W. HEROLD. V DWIGHT 0. NORTH.

REFERENCES CITED The following references are oi record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,342,896 Saltzberg Feb; 29,1944

FOREIGN PATENTS Number Country Date 415,078 Great Britain Aug. 16,1934 

